Graphene is a two-dimensional hexagonal sheet of sp2-bonded carbon atoms and has a physical strength which is at least 200 times higher than that of steel. Graphene has a thermal conductivity which is about 10 times higher than that of a metal such as copper or aluminum, and it has very high electron mobility, and thus has resistance which is at least 35% lower than that of copper at room temperature. In addition, it shows an anomalous hall effect at temperatures higher than room temperature. Thus, graphene has been reported to have interesting physical and electrical properties. Due to such properties, in recent years, studies on the preparation of high-quality graphene and on the application of graphene to devices have been actively conducted.
Graphene can generally be prepared by four methods: a mechanical exfoliation method, a chemical method employing a reducing agent, an epitaxial method employing a silicon carbide insulator, and a chemical vapor deposition (CVD) method.
A typical preparation method, first introduced in the world, is a method in which graphene is prepared from highly ordered pyrolytic graphite (HOPG) by a very delicate mechanical exfoliation technique. The mechanical exfoliation method played a crucial role in rapidly diffusing graphene research, due to simple sample preparation. However, the size of graphene prepared by the mechanical exfoliation method is only on the order of micrometers, and thus there are many limits to the actual application of the mechanical exfoliation method.
For this reason, for the preparation of large-area graphene, a method has been studied which comprises: chemically exfoliating highly ordered pyrolytic graphite (HOPG) in a liquid state by use of a strong acid to form a graphene oxide film; transferring the formed graphene oxide film onto a base substrate; and reducing the transferred graphene oxide film into graphene by a chemical reduction method. However, there is a disadvantage in that crystalline defects that can deteriorate the electrical properties of graphene can occur during the oxidation and reduction of graphene.
The epitaxial method is a method in which carbon, adsorbed on or included in crystals, grows into graphene along the surface texture of the crystals. For example, epitaxial graphene can be prepared on a SiC (0001) substrate by heat treatment in a vacuum. According to this method, a graphene film having a size similar to that of a wafer can be fabricated, but there is a problem in that the base substrate is limited to an expensive SiC (0001) substrate.
In recent years, a technique has been developed in which graphene is prepared on a catalytic metal such as nickel or copper by chemical vapor deposition with methane gas. According to the chemical vapor deposition method, it is possible to control the number of graphene layers by controlling the kind and thickness of catalyst, the reaction time, the concentration of reactive gas, etc. In addition, graphene prepared by this method have the best properties, and can be produced in large amounts.
However, when a graphene layer is formed by chemical vapor deposition, a nickel or copper catalyst layer formed for deposition of the graphene layer changes the electrical and optical properties of a base substrate on which the graphene layer is formed, and thus it also adversely affects the properties of a transparent electrode or semiconductor device comprising the graphene layer. In addition, because vapor deposition of graphene is performed at a temperature as high as about 1000° C. in order to crystallize graphene, deformation of the base substrate during the deposition process can occur when the base substrate has low heat resistance. For this reason, in order for a graphene layer formed by chemical vapor deposition to be actually used for an electrode or a device, a process of transferring a graphene layer, grown on a catalytic metal, onto a base substrate, is necessarily required.
A graphene transfer method which is generally used comprises: forming a graphene layer on a catalytic metal; removing the catalytic metal by etching using PDMS (polydimethylsiloxane) or PMMA (polymethylmethacylate) as a support layer; transferring the graphene layer onto a base substrate; and then removing the support layer. However, there is a problem in that, due to mechanical deformation (wrinkle, ripple, etc.) of graphene in the transfer process, many defects are formed at the interface between the transferred graphene layer and the base substrate, and for this reason, the behavioral characteristics of a device comprising a heterojunction of the graphene layer and the base substrate are deteriorated.
In order to overcome the problem occurring in the method of forming a graphene layer by chemical vapor deposition, various methods for increasing the success rate of transfer have been developed. The present inventors previously found that when graphene is transferred onto a Ti thin film, mechanical deformation of the graphene can be can be minimized, and thus the excellent electrical properties of the graphene can be maintained. This finding is disclosed in Korean Patent No. 10-1475460.
However, a more fundamental solution to the problem is to develop a method for forming a graphene layer, which needs no transfer process.
For this, introduction of a metal layer required to form a graphene layer by chemical vapor deposition should not affect the electrical and optical properties of a base substrate. However, a study thereon has not yet been reported.
Even though the metal layer does not affect the electrical and optical properties of the base substrate, low-temperature vapor deposition should be possible in order to form a graphene layer directly on a flexible substrate which has recently attracted attention as a base substrate for a semiconductor device. Rafik Addou et al.(Applied Physics Letters 100, 021601, 2012) reported that the use of nickel as a catalyst enables a graphene layer to be formed by chemical vapor deposition even at a low temperature of about 550° C., and also reported that carbide formed on the surface of the nickel layer inhibits the growth of graphene at a temperature below 500° C. Polyimides are being most widely used for base substrates for flexible devices, and have a glass transition temperature of about 300° C. Among these polyimides, Kapton polyimide is thermally stable at a temperature of up to about 400° C., and thus can be applied even in a relatively high temperature process. However, the temperature of the process for forming a graphene layer is still high such that chemical vapor deposition cannot be applied to a polyimide-based synthetic resin that is used for a base substrate, and the polyimide-based synthetic resin is also costly. It is strongly required to develop a base substrate having high heat resistance and to lower the process temperature so that the process can be applied even to polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyether sulfone (PES) and the like, which are inexpensive, but the application of which to base substrates is limited due to their low heat resistance.